The present invention is directed to wood composites (or composites of lignocellulosic material) formed with the use of formaldehyde-containing binder which have a low emission of formaldehyde. The invention is also directed to a process for preparing these wood composites. More particularly, the invention is concerned with improved plywood, fiberboard, particleboard and the like which are characterized by reduced formaldehyde emission, and with process for preparing them.
Urea formaldehyde condensation products are widely used as adhesives and binders. Many particleboard plants are designed around the properties of urea formaldehyde resins. They have the virtues of low cost, rapid cure, processing convenience, and clear color. Very short press cycles can be achieved with urea formaldehyde adhesives; by adding a catalyst, the rate of cure can be adjusted to essentially any desired speed. Also, urea formaldehyde adhesives have "tack", causing adhesive-treated particles to stick to each other, so that mats made from a "tacky" furnish tend to be self-sustaining in shape, facilitating handling.
Dry process composition board is a common form of composite panel. It may be made from wood fibers. In the manufacture of the board, raw wood is broken down to a fibrous form, sprayed with an appropriate adhesive, and then formed into a mat by a sifting or dry forming technique. This mat is then subjected to a high pressure and an elevated temperature to compact the mat to the desired density, commonly 40-60 lbs./ft..sup.3. In this hot pressing operation, the high temperature causes the resin to harden, to form an adhesive bond between the fibers.
In the preparation of particles used to make particleboard, a variety of materials may be employed. The board may be formed from a homogeneous type of particles. That is, all of the particles may be flakes, or all of them may be fibers. The board may be formed from a single layer or it may be multilayered, with fine surface flakes applied over a core of coarse flakes, or there may be a coarse flake core having an overlay of fibers on each of its surfaces. Other combinations are also used.
In the manufacture of particleboard, an aqueous solution of the synthetic resin binder, usually urea formaldehyde, is sprayed on the wood particles in an amount of from about 6 to about 10 parts of resin solids per 100 parts of dry wood. The resin-treated particles are then formed into a mat, and compacted in a hot press to the desired density. This type of panel is usually made to have a density in the range from about 35 to about 45 lbs./ft..sup.3. Typically, the thickness of particleboard would fall in the range from about one-eighth inch to two inches.
This type of process is quite versatile. Materials that would otherwise be waste materials can be formed into desirable products. For example, planer shavings can be formed into useful particleboard by this process, used alone, or in combination with other wood particles.
The mat process has been refined and improved, so that it is now common to make a multiple ply board. For example, three forming heads may be used. Each head effects the placement of flakes, fibers, or particles that have had resin and wax sprayed onto them, on a moving wire, or caul plate. The first forming head lays down a fine surface material, the second lays down a coarser material for the center layer of the board, and the third head lays down another outer layer of fine surface material.
Urea formaldehyde resins were developed as adhesives for wood in the late 1930's and the early 1940's. In some of the early composite panel plants, formaldehyde fumes generated during and after the hot pressing procedure were noticeable. The idea eventually arose of adding raw urea to the resin, in an effort to tie up the free formaldehyde, and reduce emissions. In some cases a limited amount of urea was added to the resin solution just prior to spraying the particles, in the commercial production of particleboard.
In addition to the mat-forming hot pressing process, an extrusion process is now in use. In this process, a mixture of wood particles, resin, and a wax size is forced through a die to make a flat board. The extrusion process is commonly used for captive production by companies who produce the resulting composite panel for use in furniture cores.
Some modern processes make use of a combination of press curing with hot platens and heat generated by radio frequency electricity. This combination permits rapid curing with a minimum press time.
While the dry process techniques for manufacturing composite panels are entirely dependent on synthetic resin adhesives, there are wet process techniques that can be used to make panels without any synthetic resin adhesive. However, often in actual practice the manufacturer of a wet process panel such as a hardboard will add a small amount of a synthetic resin binder in order to improve the properties of the product so that it can be used in demanding applications. Often the proportion of resin binder used is on the order of one-tenth to one-twentieth of the proportion used in the dry process.
In the mat-forming stage of the wet process, a slurry of fibers is drained on a screen to form a wet mat. Often the mat is produced as an endless ribbon, and it is cut into the desired panel size for curing.
In the manufacture of hardboard, the wet mat is treated somewhat differently than is the case in the dry process. The wax emulsion, for example, is added in the wet end of the mat-forming machine. Enough emulsion, generally of paraffin wax, is used to add from about 0.5% to about 1.5% of wax to the fibers, dry basis. Similarly, when a resin binder is added in the wet process, it is generally added to the fiber slurry before the mat is formed. It may be precipitated onto the fibers by acidifying the slurry with alum.
Wet process techniques are often also used in the production of insulation board. This kind of product emphasizes a low density structure that combines thermal insulating and sound-absorbing properties in a composite panel type of product. With the addition of synthetic resins and other additives, the properties such as surface quality, strength, and moisture resistance of insulation boards can be improved.
Formaldehyde release is especially noticeable in particleboard and in insulation foams, both of which contain cured resin films with a very large surface area which enhances formaldehyde release. The cause for formaldehyde release is complex. It can stem from a variety of partly related sources such as free, unreacted formaldehyde in the resin, from formaldehyde dissolved in moisture on the wood product surface, where it readily dissolves, and its vapor pressure and its release rate change with changes in air and product humidity. In particleboard, it can come from free formaldehyde which was bound to wood cellulose during the hot press cycle, and which slowly hydrolyzes under the influence of the acidic humidity in the wood. It can result from the degradation of incompletely cured resin, or resin components, such as methylolurea, and finally it can result from bulk resin degradation.
Several paths have been explored over the last few years for reducing formaldehyde release. These include coating applications, chemical treatment before or after resin application, resin additives and new resin formulations. However, relatively little research has been conducted on new resin formulations.
The mole ratio of formaldehyde to urea used commercially has tended to drop over the years, but any reductions in this ratio have weakened the internal bond character of the wood products. A new generation of low odor resins is currently appearing on the market in which traditional reagents are used, but the synthesis is more carefully controlled. Some resin formulations are now programming formaldehyde and urea additions at two or more stages in the overall reaction. Other chemicals such as resorcinol and glyoxal have been used either to terminate the dimethylolurea or to react in the polymerization process.